1. Technical Field
The present invention relates to a transmission device for converting a reciprocal linear movement to a rotary movement and more particularly to a transmission device suitable for rotating a clamp rod of a clamping apparatus or the like.
2. Description of the Earlier Technology
The present inventor proposed a clamping apparatus utilizing a transmission device of this type prior to the present invention (for example, Japanese Patent Appln. No. 8-190481). The transmission device disclosed in that previous proposal is constructed as follows.
A clamp rod has an outer peripheral surface provided with one round of a spiral groove. The spiral groove has a terminal end and an initial end communicated with each other through a communication groove in an axial direction. A sleeve externally fitted onto the clamp rod has an inner peripheral surface provided with another spiral groove corresponding to the spiral groove and with an escape groove corresponding to the communication groove. A number of rolling balls are charged between the groove of the clamp rod and that of the sleeve.
The transmission device disclosed in the previous proposal circulates the rolling balls in a peripheral direction while rolling them along the grooves when the clamp rod rotates. Therefore, it is excellent in that the frictional resistance becomes small enough to smoothly rotate the clamp rod.
However, it has been required to make the transmission device more compact so as to fulfill the recent need for downsizing the clamping apparatus.